Hybrid clocking method for single-phase transformer-free network inverters

ABSTRACT

A method for operating a transformerless inverter includes operating first and second half-bridges of the inverter using a unipolar clocking method as a first clocking method, determining a value of a grid-frequency stray current at the DC terminals of the inverter during the unipolar clocking method, and when a limit value is exceeded by the stray current value, operating the first and second half-bridges of the inverter using a stray-current-reducing clocking method as a second clocking method in which the first half-bridge provides an AC voltage at the first AC output, wherein an amplitude of the AC voltage is less than 50% of the amplitude of a voltage amplitude of the grid, and the second half-bridge provides a difference voltage between the grid voltage and the voltage provided by the first half-bridge at the first AC output.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Applicationnumber PCT/EP2017/0072394, filed on Sep. 6, 2017, which claims priorityto German Patent Application number 102016116630.8, filed on Sep. 6,2016, and is hereby incorporated by reference in its entirety.

FIELD

The disclosure relates to a method for operating an inverter and to aninverter.

BACKGROUND

Inverters are used in photovoltaic (PV) systems to convert powerprovided by solar modules as direct current to alternating current inorder to be able to feed said power into a grid. The power generatedjointly by a multiplicity of solar modules is typically converted to ACvoltage by way of an inverter, wherein transformerless inverters permitparticularly efficient conversion. In particular, transformerlessinverters that feed in a single-phase manner and have a full-bridgetopology generally do not permit grounding of the solar modules duringoperation because the potential reference with respect to groundpotential changes with the grid frequency. Said grid-frequency potentialchange, in connection with construction-dictated stray capacitances atthe solar modules, leads to stray currents, the amplitude of which canassume increased values, for example in the case of rain. PV systemshave differential current sensors for reasons of operational safety.Since the stray current is superposed with the fault current of thesystem and contributes to a fault current value measured by thedifferential current sensors, an increased stray current can lead topremature tripping of protection mechanisms based on fault currentmeasurements, as a result of which the PV system automatically switchesoff, even though safe operation would still be possible.

SUMMARY

The present disclosure is directed to demonstrate an operating methodfor a transformerless full-bridge inverter that counteracts an increasein the stray current. Accordingly, an inverter is disclosed that is lesssensitive to the influence of a high stray capacitance when determiningfault currents.

One aspect of the disclosure relates to a method for operating atransformerless full-bridge inverter comprising a first half-bridge anda second half-bridge, which are arranged in parallel with one anotherand with a link circuit between DC terminals of the inverter and therespective bridge output of which is connected by means of a filterinductor to an AC output of the inverter. The AC output is assigned tothe corresponding half-bridge, wherein the AC outputs are connected to agrid, and wherein a network of filter capacitors, which network iscoupled in a low-impedance manner to the link circuit, is arrangedbetween the AC outputs. The method comprises operating the twohalf-bridges of the inverter using a unipolar first clocking method anddetermining a value of a grid-frequency stray current at the DCterminals of the inverter. When a limit value is exceeded by the straycurrent value, the two half-ridges of the inverter are operated using astray-current-reducing second clocking method in which the firsthalf-bridge provides an AC voltage at the AC output assigned thereto,wherein an amplitude of the AC voltage is less than 50% of the amplitudeof a voltage amplitude of the grid and the second half-bridge provides adifference voltage between the grid voltage and the voltage provided bythe first half-bridge at the AC output assigned thereto. The unipolarclocking method is operated over a period whose duration corresponds tothe duration of a grid half-cycle in such a way that only one of thehalf-bridges is clocked.

The AC voltages provided by the half-bridges are, in one embodiment,each sinusoidal voltages, but any desired other voltage profiles inwhich the combination of the voltage profiles provided by the firsthalf-bridge and the second half-bridge corresponds to the voltageprofile of the grid voltage are also conceivable.

Advantageously, the amplitude of the AC voltage provided by the firsthalf-bridge is selected depending on the stray current value. Theamplitude of the AC voltage provided by the first half-bridge is, in oneembodiment, selected to be lower, the higher the stray current value is.This produces a corresponding decrease in the amplitude of the potentialfluctuation at the terminals of a generator connected to the inverter,with the result that an increase in the stray current value iscounteracted or said increase is even avoided altogether. In onespecific embodiment, the amplitude is controlled so that a maximum valueof the stray current is not exceeded.

It is advantageous in one embodiment to connect the AC output assignedto the first half-bridge to a neutral conductor of the grid and toconnect the AC output assigned to the second half-bridge to the phaseconductor. This can be ensured, for example, by virtue of the fact thatan installation instruction stipulates that one of the AC terminals isto be connected to the neutral conductor. As an alternative, theassignment can be checked once or regularly by virtue of the invertermeasuring the voltages at the AC outputs with respect to ground and, onthe basis of this measurement, determining which of the two AC outputsis connected to the neutral conductor of the grid. The assignment of thehalf-bridges present in the inverter as the first half-bridge and thesecond half-bridge can then be selected accordingly or an incorrectconnection can be indicated. As an alternative, it is also conceivable,however, that the inverter determines whether a rise or a fall in thestray current value is achieved by changing from the first clockingmethod to the second clocking method and, in the event that the straycurrent value rises, exchanges the assignment of the half-bridges of theinverter as the first half-bridge and the second half-bridge.

In addition to changing the clocking method, when the limit value isexceeded by the stray current value, the inverter can furthermoreincrease a DC voltage applied to the DC terminals by actuating aninput-side DC/DC converter. The DC/DC converter can in this case act asa boost converter or as a buck converter and, in particular, can keepthe connected solar modules at the MPP (Maximum Power Point) despite adifferent DC voltage at the DC terminals. Owing to the increased DCvoltage at the DC terminals, a higher partial voltage can be set usingthe second half-bridge so that the first half-bridge only has to set alower component of the grid voltage and hence less stray current isproduced.

In one embodiment of the method according to the disclosure, when thelimit value is exceeded by the stray current value, one of thehalf-bridges is operated in a preset manner and the other of thehalf-bridges is operated in a controlled manner. In this case, the firsthalf-bridge is operated in a preset manner. Preset operation can berealized, for example, by virtue of the fact that a prescribed clockpattern or a prescribed profile of the duty cycle is used to actuate thehalf-bridge without a voltage applied to the bridge output or a currentflowing there being taken into account. In this case, the half-bridgeoperated in a controlled manner sets the desired grid current.

In one advantageous embodiment of the method, the first half-bridge andthe second half-bridge are operated in sync with one another, that is tosay at the same clock frequency or with a fixed time reference betweenswitching times of the first and the second. Here, the switch-on timesor the switch-off times or the midpoint of the switch-on duration of twoswitches of the two half-bridges can be selected as switching times, forexample. However, it is likewise conceivable to operate the twohalf-bridges independently of one another and even at different clockfrequencies.

Since operation of the inverter in the second clocking method leads tohigher thermal loading of the switching elements and of the inductors,it is advantageous to limit a maximum inverter power to a lower valueduring operation using the second clocking method than using the firstclocking method. Stronger limitation of the maximum inverter power inthe second clocking method therefore prevents overloading of theinverter.

By taking the higher converter losses of the inverter in the secondclocking method into account, it is advantageous, when the limit valueis exceeded by the stray current value, to select an amplitude for theAC voltage provided by the first half-bridge of at most 30% of theamplitude of the grid voltage in order to achieve a reduction in thestray current value that offsets the higher converter losses of theinverter in the second clocking method. In particular, it isadvantageous to change the amplitude of the AC voltage provided by thefirst half-bridge in stages. In this case, it is advantageous to provideat least two stages in the second clocking method. As an alternative, itis likewise conceivable to prescribe an amplitude for the AC voltageprovided by the second half-bridge when the limit value is exceeded, inparticular to select said amplitude to be as great as the applied DCvoltage at the DC terminals allows, for example to select it to be halfof said DC voltage or a value reduced slightly with respect thereto.When the DC voltage at the DC terminals changes, the amplitude of the ACvoltage provided by the second half-bridge can also be adjustedcontinuously. The amplitude of the AC voltage provided by the firsthalf-bridge is adjusted accordingly in order to provide the amplitude ofthe grid voltage in total.

In order to keep the period in which the inverter has to be operatedusing the less efficient second clocking method for limiting the straycurrent as short as possible, the inverter continuously or repeatedlydetermines the present stray current value during operation using thesecond clocking method and compares it with a further limit value, whichis, in one embodiment, determined depending on the presently usedamplitude value of the AC voltage provided by the first half-bridge. Ifthe present stray current value undershoots said further limit value,this is a sign that it is possible to change back to the first clockingmethod without exceeding the limit value again. The inverter thenchanges back to the first clocking method either immediately or after aprescribed time in which the stray current value does not exceed thefurther limit again has elapsed. In particular, when the amplitude ofthe AC voltage provided by the respective half-bridges is selecteddepending on the DC voltage applied to the DC terminals, the furtherlimit value can be determined as a function of said DC voltage.

In the event that the second clocking method controls to a fixed straycurrent value, there would be a return to the first clocking method whenthe amplitude of the AC voltage provided by the first half-bridgeexceeds an amplitude limit value.

A further aspect of the disclosure relates to a transformerlessinverter, which is configured for operation using the method describedabove or the embodiments thereof. In this case, the network of filtercapacitors between the AC terminals can comprise a series circuitcomposed of two filter capacitors, the midpoint of which is connected toone of the DC terminals or to a midpoint of a link circuit designed as asplit link circuit. Furthermore, the filter inductors of the inverteraccording to the disclosure are not magnetically coupled to one another.

The two half-bridges together form, possibly with additional componentsof the inverter bridge, in particular additional switches, a H4, a H5, aH6, a H6Q or a HERIC topology. In this case, during operation using thesecond, stray-current-reducing clocking method, the additional switchesof the inverter bridge are permanently held in a switching state withina half-cycle so that the switches of the two half-bridges can providethe desired present voltage value through high-frequency clocking. Inthe H5 topology the additional switch S5 is permanently closed, in theHERIC topology the switches S5 and S6 are permanently open, and in theH6 topology the switches of the half-bridge halves having twoseries-connected switches and facing toward the DC terminals as seenfrom the bridge output are permanently closed.

In one embodiment, the inverter according to the disclosure can comprisejust one current sensor to determine the AC output current, whichcurrent sensor is arranged, in particular, at the AC output assigned tothe second half-bridge. If, during operation using the second clockingmethod, one of the half-bridges is operated in a preset manner and oneof the half-bridges is operated in a controlled manner, the currentsensor is arranged at the AC output assigned to the half-bridge operatedin a controlled manner.

In addition to the inverter bridge comprising the first and the secondhalf-bridge, an inverter according to the disclosure can furthercomprise a DC/DC converter, in particular a boost converter, the outputof which is connected to the DC terminals. In this embodiment, the DC/DCconverter can be used to provide a higher DC voltage at the DC terminalsduring operation using the second clocking method than during operationusing the first clocking method.

In a further aspect, the operating method according to the applicationrelates to a transformerless inverter, in which the inverter bridge isembodied in the H4 topology comprising a first half-bridge and a secondhalf-bridge arranged in parallel with one another and with a linkcircuit between DC terminals of the inverter. The respective bridgeoutput of the two half-bridges is connected by means of a filterinductor to an AC output of the inverter assigned to the correspondinghalf-bridge, wherein the AC outputs are connected to a grid. Here, anetwork of filter capacitors coupled in a low-impedance manner to thelink circuit is arranged between the AC outputs. The method comprisesoperating the two half-bridges of the inverter using a first clockingmethod and determining a value of a grid-frequency stray current at theDC terminals of the inverter. When a limit value is exceeded by thestray current value, the two half-bridges of the inverter are operatedusing a stray-current-reducing second clocking method. In the firstclocking method, the half-bridges are jointly operated using a bipolarclock pattern, as a result of which opposing voltage profiles with arespective amplitude of 50% of the grid voltage amplitude are providedat the two AC outputs. In the stray-current-reducing clocking method,the first half-bridge provides an AC voltage at the AC output assignedthereto of less than 50% of a voltage amplitude of the grid, or lessthan 30% of the grid voltage amplitude, and the second half-bridgeprovides a difference voltage between the grid voltage and the voltageprovided by the first half-bridge at the AC output assigned thereto. Inthe first clocking method, this produces a fluctuation of the voltage ofthe DC terminals with respect to ground potential of 50% of the gridvoltage amplitude and, in the second clocking method, a fluctuationreduced in comparison therewith and, as a result, a reduced value of thegrid-frequency component of the stray current. The amplitude of thevoltage provided by the first half-bridge is, in one embodiment,selected depending on the level of the stray current, in particular isselected so that the grid-frequency component of the stray currentremains below a prescribed critical value.

Said amplitude is, in one embodiment, changed in stages whencorresponding limit values of the grid-frequency component of the straycurrent are exceeded or undershot. The amplitude is in this case loweredwhen corresponding limit values are exceeded and increased when they areundershot.

Said alternative operating method according to the application can beused, in particular, in connection with semiconductor switches, forexample made of gallium nitride, that switch more rapidly compared tosilicon switches, as a result of which the switching and magnetizinglosses during the second clocking method are not or are notsignificantly increased compared to the first clocking method.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure and some of the design variants thereof are explained inmore detail in the following text with the aid of figures, wherein

FIG. 1 shows a schematic design of an inverter according to thedisclosure in the H4 topology,

FIG. 2 shows a further embodiment of an inverter according to thedisclosure in the H5 topology,

FIG. 3 shows a time profile of voltages during operation of the inverteraccording to the disclosure in a unipolar first clocking method, and

FIG. 4 shows a time profile of voltages during operation of the inverteraccording to the disclosure in a stray-current-reducing second clockingmethod.

DETAILED DESCRIPTION

The inverter 1 shown in FIG. 1 has DC terminals 2, 3 to which a voltagesource (not shown), in particular a PV generator, can be connected. Alink circuit DCL is arranged between the DC terminals 2, 3, and a firsthalf-bridge HB1 and a second half-bridge HB2 are arranged in parallelwith the link circuit DCL. The first half-bridge HB1 can be formed fromtwo series-connected semiconductor switches T3, T4. The midpoint of thesemiconductor switches is led out of the first half-bridge HB1 as afirst bridge output Br1. Analogously, the second half-bridge HB2 can beformed from two series-connected semiconductor switches T1, T2, themidpoint of which is led out of the second half-bridge HB2 as a secondbridge output Br2. The semiconductor switches can have an intrinsic or aseparate antiparallel freewheeling diode.

A first filter inductor L1 connects the first bridge output Br1 to afirst AC output AC1, a second filter inductor L2 connects the secondbridge output Br2 to a second AC output AC2. A network 4 of filtercapacitors, which, together with the filter inductors L1, L2, form an ACgrid filter, is arranged between the two AC outputs AC1, AC2. Thenetwork 4 is formed here by way of a series circuit composed of twofilter capacitors, the midpoint of which is connected to the DC terminal3 by means of a low-impedance connection 6. The low-impedance connection6 is in this case a direct connection, wherein it is likewiseconceivable to provide in the connection further component parts havinga low impedance at the grid frequency and at the switching frequency ofthe bridges. As an alternative to the DC terminal 3, the midpoint of thenetwork 4 can likewise be connected to the DC terminal 2 or the midpointMP of a link circuit DCL, which is split in this case, so that therespective potentials are coupled to one another. The first AC outputAC1 can be connected to a neutral conductor N of a grid and the secondAC output AC2 is connected to a phase conductor L of said grid. It isconceivable that the grid filter has further filter components, inparticular further filter inductors between the network 4 and theconnected grid.

The two half-bridges HB1, HB2 are actuated by way of controllers C1, C2assigned thereto. The controller C1 switches on the switches T3, T4 ofthe first half-bridge HB1 by means of pulse-width modulation, thecontroller C2 switches on the switches T1, T2 of the second half-bridgeHB2. The current provided by the half-bridges is detected by currentsensors CS arranged at the bridge outputs Br1, Br2. In this embodiment,a value of the stray current can be determined by determining thedifference between the measurement values of the two current sensors CS.In this case, the grid-frequency stray current is the frequencycomponent of the difference at the frequency of the connected grid. Asan alternative, the grid-frequency stray current can also be determinedby way of further sensors on the AC side or the DC side of the inverter1 in a manner already known.

FIG. 2 shows a further embodiment of an inverter 1 according to thedisclosure. In this case, a PV generator PV is connected to the DCterminals 2, 3 by means of a DC/DC converter BC, for example a boostconverter. In the other inverter topologies as well, such a DC/DCconverter BC can be provided on the input side in order to convert thevoltage of the PV generator PV to the voltage of the link circuit DCL. Astray capacitance 7 is furthermore shown symbolically as a cause of astray current, which stray capacitance connects the PV generator PV toground GND. The inverter bridge is designed in this case as what isknown as a H5 bridge, which, in addition to the transistors T1 to T4 ofthe two half-bridges HB1, HB2, also comprises a further transistor T5,which connects the upper connection point of the half-bridges HB1, HB2to the DC terminal 2. The link circuit DCL is in this case likewiseembodied as a split link circuit with a midpoint MP. The network 4 ofthe AC filter is provided on the output side with, in addition to aseries circuit of two filter capacitors between the terminals L, N towhich the grid 5 is connected, a further capacitor as well, whichfurther capacitor is arranged directly between the two grid terminals L,N. The midpoint of the series circuit composed of the filter capacitorsof the network 4 is in this case connected directly to the midpoint MPof the link circuit DCL. In the embodiment shown here, only one singlecurrent sensor CS is provided in the connection lines to the grid 5,which is also likewise possible in the other conceivable embodiments.

The additional transistor T5 in the H5 topology serves to electricallyisolate the connected PV generator PV from the connected grid 5 duringthe freewheeling phases. In the context of this disclosure, saidtransistor performs this function but only during operation using thefirst, unipolar clocking method. During operation using the secondclocking method, T5 remains permanently switched on and hence makes itpossible to independently operate the two half-bridges HB1, HB2 at thelink circuit DCL.

To explain the functioning of the disclosure in more detail, FIG. 3first of all shows a temporal profile of voltages, as arises, forexample, from the H5 topology from FIG. 2, when it is operated in thefirst clocking method. The grid voltage U₀ has the known sinusoidalprofile with an amplitude Û₀. In the unipolar clocking method, duringone half-cycle of the grid voltage, only one of the two half-bridgesHB1, HB2 is clocked. In connection with the potential-free freewheeling,which arises in a manner dependent on the topology by way of theclocking of further switches, the half-bridges generate at the ACterminals AC1, AC2 mutually opposing sinusoidal profiles U_(AC1MP),U_(AC2MP) with half the grid amplitude Û₀/2 based on the potential ofthe link circuit midpoint MP. Said two sinusoidal profiles are addedtogether to form the profile of the grid voltage U₀. Since the ACterminal AC1 is fixedly connected to the N conductor of the grid 5, thepotential of the link circuit midpoint MP varies with respect to groundpotential, which is assumed here as equal to the potential of the Nconductor for simplification, likewise sinusoidally with an amplitudecorresponding to half the grid amplitude U₀/2. This leads to agrid-frequency component of a stray current across the stray capacitance7 that is proportional to the amplitude of the variation of thegenerator potential. Said component is added to a fault current flowingon account of a non-ideal isolation of the PV generator PV and can lead,in particular in the case of a high value of the stray capacitance 7, totripping of an isolation monitoring circuit, even though the system isstill in a sufficiently isolated state. In order to reduce thegrid-frequency component of the stray current, it is therefore desirableto reduce the grid-frequency amplitude of the variation of the generatorpotential in order to likewise reduce the corresponding grid-frequencycomponent of the stray current.

Here, the second clocking method according to the present disclosurebecomes important, which method is able to reduce said amplitude of thevariation of the generator profile to a lower value than half the gridamplitude Û₀/2. The functioning of said second clocking method isillustrated and explained with the aid of the voltage profiles shown inFIG. 4.

To this end, FIG. 4 again shows the sinusoidal profile of the gridvoltage U₀ in comparison with the voltage profiles U_(AC1MP), U_(AC2MP)at the respective AC terminals AC1, AC2 with respect to the potential ofthe link circuit midpoint MP. The amplitude Û_(AC1MP) of the voltageprofile at the AC terminal AC1 is in this case lower than the amplitudeÛ_(AC2MP) of the voltage profile at the AC terminal AC2. The amplitudeÛ_(AC1MP) is advantageously at most 30% of the grid voltage amplitudeÛ₀. Correspondingly, the amplitude Û_(AC2MP) is at least 70% of the gridvoltage amplitude Û₀. Since, as described already, the AC terminal AC1is connected to the neutral conductor of the grid 5, the voltage profileof the link circuit midpoint MP varies only with the smaller amplitudeÛAC1MP and is accordingly reduced with respect to the amplituderesulting from the first clocking method described above. Analogously,the grid-frequency component of the stray current is also reduced by thesecond clocking method.

In order to achieve the asymmetrical voltage profiles at the ACterminals AC1, AC2, the respective half-bridges HB1, HB2 are controlledindependently of one another to the voltage profiles of correspondingtarget values of the bridge voltage. As an alternative to voltageregulation in the half-bridges HB1, HB2, it is also possible for justone half-bridge, for example, the first half-bridge HB1, to be operatedin a voltage-controlled manner, whereas the second half-bridge isoperated in a current-controlled manner, so that a desired grid currentis produced. The voltage regulation can be carried out by determiningthe deviation of a measured voltage at the respective AC terminals AC1,AC2 from the prescribed target values and a change in the respectiveduty cycle that can be used to operate the half-bridges HB1, HB2, whichchange corresponds to the deviation. However, it is also conceivable tooperate one of the two half-bridges, for example, the first half-bridgeHB1, using a predefined clock pattern, which leads at leastapproximately to a voltage profile at the AC terminal AC1 that has thedesired amplitude Û_(AC1MP). In this case, it is only necessary tooperate one of the two half-bridges in a controlled manner.

The first half-bridge HB1 can be operated independently of the secondhalf-bridge HB2, that is to say, in particular, even at a deviatingfrequency. In this case, there is no temporal correlation between theswitching times of the bridge switches of the two half-bridges. However,it is likewise easily possible to operate the two half-bridges at thesame clock frequency and, in particular, in sync with one another, forexample by virtue of the midpoints of the switch-on periods of the firstand the second half-bridge being synchronized.

In order to provide the asymmetrical voltage profiles at the ACterminals AC1 and AC2, the two half-bridges have to be clockedpermanently, that is to say both switches of the half-bridges arealternately closed during both half-cycles so that both the positive andthe negative voltage of the DC terminals 2, 3 is provided in phases atthe bridge output of said half-bridges. This is the cause for theincreased converter losses described above during operation using thesecond clocking method in comparison with operation using the firstclocking method.

The minimum voltage U_(DCmin) of the generator required in the second,stray-current-reducing clocking method is determined by the amplitudeÛ_(AC2MP) at the second AC terminal AC2 and is twice this value. Saidvalue is higher than the required minimum voltage U_(DCmin) of thegenerator when the inverter 1 is operated using the first clockingmethod (U_(DCmin)=Û₀, see FIG. 3). It is therefore advantageous or evennecessary to increase the generator voltage when a change is made fromthe first to the second clocking method in order to preventundershooting of said minimum voltage. The generator voltage can beadjusted by way of a DC/DC converter BC connected upstream of theinverter.

The invention claimed is:
 1. A method for operating a transformerlessinverter comprising a first half-bridge and a second half-bridgearranged in parallel with one another and with a link circuit betweenfirst and second DC terminals of the inverter, wherein each of the firsthalf-bridge and the second half-bridge have an output of which isconnected by means of a respective filter inductor to first and secondAC outputs of the inverter, respectively, such that each AC output isassigned to the corresponding half-bridge, wherein the first and secondAC outputs are connected to a grid, and wherein a network of filtercapacitors coupled in a low-impedance manner to the link circuit isarranged between the first and second AC outputs, comprising: operatingthe first and second half-bridges of the inverter using a unipolarclocking method as a first clocking method, determining a value of agrid-frequency stray current at the DC terminals of the inverter duringthe unipolar clocking method, and when a limit value is exceeded by thestray current value, operating the first and second half-bridges of theinverter using a stray-current-reducing clocking method as a secondclocking method in which the first half-bridge provides an AC voltage atthe first AC output, wherein an amplitude of the AC voltage is less than50% of the amplitude of a voltage amplitude of the grid, and the secondhalf-bridge provides a difference voltage between the grid voltage andthe voltage provided by the first half-bridge at the first AC output. 2.The method as claimed in claim 1, wherein the AC voltages provided bythe first and second half-bridges are each sinusoidal voltages.
 3. Themethod as claimed in claim 1, wherein the amplitude of the AC voltageprovided by the first half-bridge in the stray-current-reducing clockingmethod is selected depending on the stray current value, wherein theamplitude is selected to be lower given a higher stray current value. 4.The method as claimed in claim 1, wherein the amplitude of the ACvoltage provided by the second half-bridge is selected depending on avoltage applied to the DC terminals.
 5. The method as claimed in claim1, wherein the AC output assigned to the first half-bridge is connectedto a neutral conductor of the grid.
 6. The method as claimed in claim 1,wherein, when the limit value is exceeded by the stray current value, aDC voltage applied to the DC terminals is further increased by actuatingan input-side DC/DC converter of the inverter.
 7. The method as claimedin claim 1, wherein, when the limit value is exceeded by the straycurrent value, one of the first and second half-bridges is operated in apreset manner and the other of the first and second half-bridges isoperated in a controlled manner.
 8. The method as claimed in claim 1,wherein the first half-bridge and the second half-bridge are operated insync with one another.
 9. The method as claimed in claim 1, wherein thefirst half-bridge and the second half-bridge are operated independentlyof one another, in particular at different clock frequencies.
 10. Themethod as claimed in claim 1, wherein a maximum inverter power of theinverter is limited to a higher value during operation using the firstclocking method than using the second clocking method.
 11. Atransformerless inverter comprising a first half-bridge and a secondhalf-bridge arranged in parallel with one another and with a linkcircuit between first and second DC terminals of the inverter, whereineach of the first half-bridge and the second half-bridge have an outputwhich is connected by means of a respective filter inductor to first andsecond AC outputs of the inverter, such that each AC output is assignedto the corresponding half-bridge, wherein the first and second ACoutputs are connected to a grid, and wherein a network of filtercapacitors coupled in a low-impedance manner to the link circuit isarranged between the AC outputs, configured for operation using amethod, comprising: operating the first and second half-bridges of theinverter using a unipolar clocking method as a first clocking method,determining a value of a grid-frequency stray current at the DCterminals of the inverter during the unipolar clocking method, and whena limit value is exceeded by the stray current value, operating thefirst and second half-bridges of the inverter using astray-current-reducing clocking method as a second clocking method inwhich the first half-bridge provides an AC voltage at the first ACoutput, wherein an amplitude of the AC voltage is less than 50% of theamplitude of a voltage amplitude of the grid, and the second half-bridgeprovides a difference voltage between the grid voltage and the voltageprovided by the first half-bridge at the first AC output.
 12. Theinverter as claimed in claim 11, wherein the network comprises a seriescircuit composed of two filter capacitors, the midpoint of which isconnected to one of the first and second DC terminals or to a midpointof a link circuit designed as a split link circuit.
 13. The inverter asclaimed in claim 11, wherein the filter inductors of the AC outputs arenot magnetically coupled.
 14. The inverter as claimed in claim 11,wherein the first and second half-bridges are coupled with additionalcircuitry to collectively comprise a bridge topology comprising one ofthe bridge topologies: H4, H5, H6, H6Q and HERIC.
 15. The inverter asclaimed in claim 11, wherein just one current sensor (CS) is provided todetermine the AC output current, which current sensor is arranged at thesecond AC output assigned to the second half-bridge.
 16. The inverter asclaimed in claim 11, further comprising a DC/DC converter connected tothe first and second DC terminals.